This invention generally relates to improvements in reciprocating pumps. More specifically, the present invention relates to an improved piston for such pumps. Although the invention will be described with particular reference to a pump piston used with slush or mud pumps, it will be recognized that certain features thereof may be used or adopted to use in other types of reciprocating pumps.
Slush or mud pumps are used in connection with oil well drilling operations for pumping drilling mud. Such pumps typically operate at high pressures due to the necessity for pumping the drilling mud through perhaps several thousand feet of drill stem. Moreover, it is necessary for the mud to emerge from the drill bit at a relatively high velocity in order to provide lubrication and cooling to the bit, and in order to provide a vehicle for the removal of drill cuttings from the earth formation being drilled.
The pistons and cylinders used for such mud pumps are susceptible to a high degree of wear during use because the drilling mud is of high specific gravity, and has a high proportion of suspended and gritty abrasive solids which can be quite erosive. As the pump cylinder becomes worn due to erosion, the small annular space between the piston and the cylinder wall will increase substantially. Because of this result, the seal design for such pumps is critical.
The high pressure abrasive environment in which the pumps must operate is especially deleterious to the seals since considerable friction is generated, and since the hydraulic pressures encountered force the seal into the annular space between the cylinder wall and the piston. Indeed, the frictional forces may even detach the seal from the piston. With piston movement, the edges of a conventional seal become damaged very quickly by the cutting or tearing action that occurs to the seal material. Another problem with conventional mud pump seals is that they do not adequately "wipe" the cylinder wall so that the pressurized drilling mud thus seeps between the seal and the cylinder wall.
Attempts have been made to securely fasten the seal in the piston to resist this frictional force. One conventional solution to this problem has been use of a metallic seal retainer which is disposed over the seal body and retained in place by snap rings. However, one disadvantage to this solution is that the additional seal retaining element and its snap rings render the overall piston construction more expensive. A further disadvantage is that the seal is made somewhat less flexible and resilient than it would otherwise be, thus decreasing its ability to wipe the cylinder wall effectively.
Another previously proposed solution to the problem has been the use of a sealing element which has an interlocking relationship with the piston hub structure. One disadvantage of this system has been the precise nature of the interlocking construction employed. That is, the interlocking construction is generally triangular in shape and does not provide sufficient surface area to afford a sturdy interlock connection. Moreover, the piston seal does not have a separate, resilient cylinder wall contacting leg or lip.
Although resilient wall contacting lips or legs are known for piston seals used in air or hydraulic brake systems and in stationary joints in water pumps, they are not adapted for use in the high pressure abrasive environment of slush or mud pumps. One reason for this is due to the fact that the design parameters are such that the seal rings would not be prevented from being dislodged from the piston under the high friction conditions encountered in slush or mud pumps. Another reason is due to the fact that in brake systems, for example, the gap between the wall contacting leg and inner leg is not designed to prevent collection of large amounts of particulates since such particulates are not present in these systems. Thus, if such seals were used on slush pumps, the effectiveness of the seal would quickly be diminished.
It has, therefore, been considered desirable to develop a new and improved reciprocating pump piston which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.